Two polymorphs of a new AIEgen from transition-metal-free cross-coupling reactions: A combined experimental and crystal structure prediction study

Abstract

We synthesized a new double aromatic pyrrole [2,5-bis(2,5-dimethoxyphenyl)-1H-pyrrole, abbreviated as BDMP] without using transition metals. This compound exhibits a conjugated skeleton, ideal for aggregation-induced emission luminogens (AIEgens). Two methods, Lewis acid catalysis and photoreaction, were used for synthesis. Different methods produce different crystal structures with the same molecular skeleton. The crystals show different AIE properties. The photocatalytically formed crystal (BDMP-B) is highly luminescent with a 10.4 % fluorescence yield. In contrast, the Lewis acid-catalyzed crystal (BDMP-A) has a lower fluorescence yield of 3.0 %. These differences in emission are due to varying crystal packings, confirmed by quantum calculations.To understand these differences, Crystal Structure Prediction (CSP) studies were conducted. Our goal was to find if more stable crystal packings exist. Several energy ranking approaches were used to identify the most feasible polymorphic forms. These approaches include static lattice energy, lattice binding energy, enthalpy, cohesive enthalpy, Gibbs Free Energy, cohesive Gibbs Free Energy, and high-accurate electronic binding energy from large molecule-cluster crystal packings. Our findings suggest that the most stable crystal forms can be predicted accurately using appropriate theoretical methods.This research improves our understanding of AIEgens. It shows the value of combining experimental work with both cost-effective and highly accurate theoretical methods for crystal structure analysis and prediction.